Quantum communications have now been deployed and field-tested over a large diversity of optical networks, including metropolitan and regional fiber networks, but also in the context of optical communications in space. Many challenges remain to be addressed as to evolve towards quantum with the ability to interconnect nodes with quantum computing or quantum sensing capabilities. Cost-effectiveness, however, appears as another key challenge for quantum networks that can now be tackled effectively. It typically consists in designing and manufacturing quantum communication systems leveraging affordable technologies, but also in integrating them within already existing infrastructures, and learning how to mutualize network resources.

In their work, Guihua Zeng and coauthors propose an ingenious continuous-variable quantum key distribution (CV-QKD) system design, that relies on a round-trip architecture and enables to use the same low-linewidth laser to generate multiple CV-QKD channels, based on subcarrier frequency modulation at the MHz level. In addition to providing a proof-of-principle experimental demonstration of such a multi-user QKD architecture, typical of access networks, a detailed noise analysis illustrates its scalability. This paper makes a notable contribution to demonstrate both the feasibility and relevance of the development of cost-effective quantum networks, a crucial milestone on the path towards their wide-scale deployment.

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